Hitherto, regarding a method of providing differences in thickness in the direction of width of a strip, gthere has been known a method of using rolls each provided thereon with a crown or another method of using a roll provided thereon with a groove-like caliber.
The former pertains to an operation of producing usual strips. According to this method, in a case of providing a large difference in thickness, rolls each having a large crown are used in preceding stages of rolling regarding a hot strip mill, and the size of each of the crowns is reduced in accordance with the successive reduction of thickness of a rolled material, thereby forming a crown on the strip without disturbing a contour of the strip. In the case of this method, a widthwise cross-sectional shape of a strip to be formed is limited to a shape of the crown.
In the latter method, a groove is formed in a roll so as to provide a local projection on a strip. For example, this type of method is disclosed in U.S. Pat. No. 3,488,988, Japanese Examined Patent Publication No. 34022/1977, Japanese Unexamined Patent Publication No. 88943/1980 and so forth. Since, in this method, a provision of the thickness difference is effected in a final stand, the shape of a strip will be degraded if a difference in thickness is large between a section defined prior to the rolling which is to be effected at the final stand and another section provided by the rolling in the final stand of the mill, with the result that a defective product will be caused or it will become impossible to effect the rolling at the final stand. Thus, in the method there is a limitation regarding the section which can be imparted to strips. This rolling method is effective when applied to the rolling of a steel strip of deformed section in which a variation in thickness is repeated widthwise with a relatively small pitch. However, in such a case where a pitch "P" showed in FIG. 1 is large and where a thickness of the whole of a steel strip (,i.e., both of a thickness t.sub.1 of a thick portion and another thickness t.sub.2 of a thin portion) is relatively small, that is, in a case where a steel strip of deformed section is of a thin strip shape, a shape of a resultant steel plate has been apt to be degraded with the result that a defective product or defective rolling has been apt to be caused. FIG. 5 illustrates an example of rolling for simultaneously obtaining a pair of deformed section steel strips each similar to that shown in FIG. 1 in which rolling a roll barrel of a rolling mill is effectively utilized to simultaneously provide two sets of deformed section in a steel strip. In the drawing the reference numerals 1 and 3 denote upper and lower working rolls for effecting the rolling to obtain the deformed section steel strip, respectively. Reference numeral 2 denotes a caliber provided on the upper roll 1, and reference numeral 4 denotes a material to be rolled.
The present applicant has already disclosed, in the specification of Japanese Patent Application No. 128880/1984 (Japanese Unexamined Patent Publication No. 9911/1986), a method of rolling by use of roll having caliber so as to produce a good steel strip of a deformed section having a large difference in thickness in the direction of the width thereof, in which method a continuous hot rolling mill having one or a plurality of stands is used. This rolling method of producing a deformed section strip is characterized in that the strip is rolled under the following rolling condition for each stand: EQU .vertline.Ch.sub.E /h.sub.E -Ch.sub.D /h.sub.D .vertline..ltoreq.0.3
wherein Ch.sub.E is a thickness difference (mm) of a deformed section defined at the entrance side; h.sub.E is an average thickness (mm) thereof at the entrance side; Ch.sub.D is a thickness difference (mm) of another deformed section defined at the exit side; and h.sub.D is an average thickness (mm) thereof at the exit side.
The minimum conditions for producing by use of rolls, a deformed section steel strip widely varying in thickness were established in the specification of the above-mentioned Japanese Patent Application No. 128880/1984. However, there has been no effective measure for preventing a rolled steel strip from being biased widthwise when the strip is rolled, and a further improvement has therefore been desired.
That is, in the case of rolling a strip, a biasing has occurred regarding the position of the strip since there is no mechanism for retaining strips widthwise in place regarding the transverse position thereof. However, in a case of rolling a flat-rolled steel strip, a widthwise slight biasing substantially causes no problem unless the steel strip is in a disengaged relation with a roll barrel. A bias occurring in another case of rolling a steel strip of deformed section having a thickness difference in the direction of width causes a position for engagement with rolls of a next stand to be deviated from a predetermined correct position, with the result that a difference in rolling reduction ratio occurs in the direction of the width of the steel strip, with an elongation difference also occurring to cause an improper shape, so that in an extreme case it becomes impossible to effect the rolling due to the occurrence of defects such as bore or due to the occurrence of a phenomenon of chew up. FIG. 6 illustrates a main part of a strip and rolls at the time of occurrence of a biasing in this rolling process. In particular, when the strip is to be produced in a multiple-stage rolling manner, the influence of such biasing is more significant. In FIG. 6, a reference numeral 5 indicates the center of a convex portion of the roll, a reference numeral 6 indicating the center of a recessed portion of a rolled strip, a reference numeral 7 indicating the extent of the bias regarding a strip. A reference symbol A indicates an area of the strip at which the strip is subjected to excessive rolling reduction, and a reference symbol B indicates an area where the strip is subjected to insufficient rolling reduction. Thus, a difference in the rolling reduction occurs between the areas A and B, which causes a difference in the elongation in the longitudinal direction, resulting in local shape defects of a rolled product or, in an extreme case, making it impossible to effect rolling.
Accordingly, in the case of a multistage rolling of this type of deformed section steel strip it is essential for a steel strip to stably pass continuously the center of each of stands provided with caliber, under an optimal rolling schedule so that a desired contour of a rolled product may be obtained.
Certain conditions necessary for the rolling of steel strips having a wide range of difference in thickness were disclosed in the specification of the above-mentioned previously filed Japanese Patent Application No. 128880/1984, but these conditions alone are insufficient, and the establishment of further conditions has been desired.
That is, a shape, an influence of the thickness difference at the entrance side of a rolling stand, on the thickness difference at the exit side thereof, and influence of the depth of a caliber of a rolling stand on the thickness difference at the exit side have needed to be examined in detail. For example, when obtaining a predetermined thickness difference by rolling, it is necessary to examine whether or not the aimed thickness difference can be attained by a caliber-roll pass at one stand, or it is necessary to determine optimum values such as the number of necessary stands in the case of multi-stand rolling by use of rolls having caliber.